(1) Field of the Invention
The present invention relates generally to a pressure monitoring system for a vacuum circuit interrupter, and particularly to a pressure monitoring system using an optical device to detect the change in vacuum pressure within an evacuated envelope of a high-voltage vacuum circuit interrupter having a plurality of capacitors connected in series with one another, the whole group of capacitors being connected in parallel with the vacuum circuit interrupter, located outside the evacuated envelope to divide the voltage applied to the interrupter equally between each capacitor.
(2) Description of the Prior Art
In general, it is necessary to monitor vacuum pressure within a vacuum related electrical apparatus such as a vacuum circuit interrupter since the performance of such a vacuum circuit interrupter depends on whether the degree of vacuum pressure within an evacuated envelope is lower than10.sup.-4 Torr as indicated in column 1 lines 11-13 of U.S. Pat. No. 4,034,264.
The prior art of the pressure monitoring system is briefly described hereinafter. The pressure monitoring system for such a vacuum circuit interrupter is already proposed by the applicant in Japan patent application No. 55-37098. The disclosed pressure monitoring system comprises a light source, a polarizer for linearly polarizing the light from the light source, an electric field detecting element such as a Pockel's cell utilizing the Pockel's effect that changes the angle of the polarization plane with respect to that of the incident light from the polarizer according to the electric field intensity applied thereto thus changing in response to vacuum pressure within the vacuum circuit interrupter, an analyzer having its polarization plane in a predetermined relationship with that of the polarizer, for receiving the light from the Pockel's cell, and a light receiving member for receiving the light incident from the analyzer and photoelectrically converting it into an electrical signal.
When the pressure monitoring system of the type described above is installed in the vicinity of the vacuum circuit interrupter, the deterioration of vacuum pressure can be monitored without electrically touching the interrupter since vacuum pressure within the vacuum circuit interrupter is substantially proportional to the electric field intensity in the vicinity of the vacuum circuit interrupter (that is, in the space near the outside of the evacuated envelope of the vacuum circuit interrupter where the electric field intensity changes with vacuum pressure).
High-voltage large-sized vacuum circuit interrupters are provided with a plurality of capacitors in series between an end plate connected to a stationary electrode holder supporting a stationary electrode contact, an arc shielding member, and another end plate connected to a movable electrode holder. These capacitors are used to divide the voltage applied to the electrode contacts equally between each capacitor so that the interruption performance can be improved.
Conventionally, the monitoring of vacuum pressure in the vacuum circuit interrupter of the type as described above needs to be performed after modifying the construction of the vacuum circuit interrupter of the type where the capacitors are connected in parallel therewith so as to detect vacuum pressure therein.